This invention relates to protective coatings on substrates, and, more particularly, to a composite protective coating formed of a mixture of a metal and a ceramic.
In an aircraft gas turbine (jet) engine, air is drawn into the front of the engine, compressed by a shaft-mounted compressor, and mixed with fuel. The mixture is burned, and the hot exhaust gases are passed through a turbine mounted on the same shaft. The flow of gas turns the turbine, which turns the shaft and provides power to the compressor. The hot exhaust gases flow from the back of the engine, driving it and the aircraft forwardly.
The hotter the exhaust gases, the more efficient is the operation of the jet engine. There is thus an incentive to raise the exhaust combustion gas temperature, which in turn leads to higher operating temperature requirements of many of the components from which the engine is constructed. In response to these requirements, alloys with improved mechanical properties have been developed for use in the various sections of the engines.
It has been the conventional practice to use nickel-base and cobalt-base alloys for components that operate at high temperatures, and additionally for components operating at intermediate temperatures. These alloys are relatively dense, and there has been an ongoing effort to substitute less-dense materials in intermediate-temperature components. Among these substitute materials are titanium-base alloys being developed for use in high-pressure compressor blades and vanes, as well as other portions of the engine structure. Such components typically operate at temperatures of from about 1000xc2x0 F. to about 1800xc2x0 F. They are not cooled by internal cooling or other mechanisms.
Even at these intermediate operating temperatures there can be extensive damage to the engine components due to oxidation, hot corrosion, hot salt stress corrosion, and other mechanisms. To inhibit such damage, environmental coatings for the uncooled components have been developed. Unlike thermal barrier coatings used on cooled components such as high-pressure turbine blades and vanes, these environmental coatings do not provide an insulating function, but instead are designed solely to reduce the incidence of damage to the uncooled component.
The available environmental coatings for uncooled titanium-base gas turbine engine components are metallic alloys that form a protective oxide scale over the substrate to which they are applied. These environmental coatings, while operable, can separate from the substrate during the thermal cycling experienced in extended service. Some may also chemically react with the highly reactive titanium-base substrate during service to form undesirable phases which themselves degrade the performance of the component.
There is a need for an improved approach to protecting uncooled substrates made of titanium-base alloys, which achieves good environmental protection of the substrates without the disadvantages found in current environmental protective coatings.
The present invention provides a coating and a titanium-base alloy article that is coated with the coating to protect it during elevated temperature service in adverse environments. The coating provides excellent protection against diffusionally based oxidation as well as hot corrosion, hot salt stress corrosion, hot erosion, and other damage mechanisms. Reaction of the coating with the titanium-base substrate is minimal. The coating bonds well to the substrate. The coating is designed to avoid the creation of excessive thermal strains and stresses that would otherwise be present due to the difference in thermal expansion coefficients of the coating and the metallic substrate. Consequently, no weight-adding bond coat is required between the protective coating and the substrate. The protective coating can be applied to the substrate using well established procedures such as thermal spraying techniques.
In accordance with the invention, an article comprises a substrate formed of a first metal comprising a titanium-base alloy, and a coating directly in contact with the substrate. The coating is formed of a mixture of a second metal and a ceramic. The second metal is preferably an environmentally resistant metal such as a nickel-base alloy or a cobalt-base alloy. The ceramic is preferably an aluminum-bearing, chromium-bearing, or silicon-bearing ceramic, most preferably an oxide. The second metal and the ceramic are selected to impart desired properties to the coating. For example, the second metal is an alloy that is selected for its excellent corrosion or oxidation resistance, and the ceramic can be selected for its high impermeability to oxygen diffusion, low thermal conductivity, and/or low thermal expansion. These properties are reflected in the mixture.
The substrate has a substrate coefficient of thermal expansion, and the coating mixture has a mixture coefficient of thermal expansion. In the preferred approach, the mixture coefficient of thermal expansion is about the same as that of the substrate coefficient of thermal expansion over at least some temperature range. To this end, in the preferred approach the thermal expansion coefficient of the substrate is determined, and a coating proportion of the second metal and the ceramic is selected such that the thermal expansion coefficient of the mixture that forms the coating is about the same as the thermal expansion coefficient of the substrate.
In the composite coating mixture the beneficial properties of the individual constituents are retained. The coating can therefore be designed to have good protective characteristics by virtue of the selection of the constituents. The coefficient of thermal expansion of the coating is related to the relative amounts of the metallic constituent, which has a high coefficient of thermal expansion, and the ceramic constituent, which has a low coefficient of thermal expansion. Consequently, the relative amounts of the constituents are selected so that the thermal expansion coefficient of the coating approximately matches that of the substrate, minimizing the likelihood of debonding of the coating from the substrate during thermal processing or service. The composite mixture structure of the coating also reduces its susceptibility to cracking and fatigue cracking, inasmuch as the metal matrix of the coating serves to inhibit crack propagation through the ceramic material of the coating. Lastly, the presence of the ceramic in the coating reduces the amount of contact area between the metallic component of the coating and the surface of the substrate, reducing the extent of available surface area over which adverse reactions, if any, between the metallic component of the coating and the surface of the substrate can occur.